This invention relates to chemical process equipment in which a liquid is contacted with a counterflow of gas. This may be for a variety of purposes such as stripping a component from the liquid stream or absorbing a component into a liquid stream. More generically this invention relates to equipment designed to facilitate mass and/or heat transfer between phases.
The type of equipment to which this invention specifically relates employs cross-flow fractionation trays connected by downcomers. In such equipment a tower is provided with a plurality of fractionation trays arranged generally horizontally within the tower. Each tower has a perforated deck and at least one channel, called a downcomer, in which a liquid flowing over the deck may be collected and channeled to the tray below. In use a gas or vapor is introduced at the base of the tower and passes upwards through the perforations in the decks of the fractionation trays. Meanwhile a liquid is introduced at the top of the tower and percolates downward passing over the fractionation trays and down the downcomers to the tray below.
Upon reaching the tray, the liquid flows across the tray in what is described here as the "design flow direction", which indicates the direction the liquid is intended to flow when the tray is operating in optimum design conditions. The tray is provided with a plurality of perforations through which the gas bubbles continuously at a pressure that, under normal operating conditions, precludes the liquid from passing through the perforation. These perforations, in conjunction with the associated covers, are referred to as "valves" and are designed to permit efficient mass transfer contact between the gas and the liquid. These valves fall into two major groups: fixed and movable. The fixed valves have no moving parts and are properly called "deflectors". The movable are adapted to respond to the pressure of the up-flowing gas by opening or closing the valve. The present invention relates to movable valves.
In the ideal process design, the liquid should be prevented from passing through the valves in the trays by the pressure of gas passing through the perforations in the upward direction. This is a finely balanced process since, if the pressure is too great, the gas will have a shorter transit time within the tower and less efficient contact with the liquid as it flows across the trays and down the tower. The high gas velocity may also cause liquid droplets to be carried up to the tray above, thereby reducing the separation efficiency as a result of back-mixing. On the other hand if the gas flow rate is too low the liquid will penetrate through the valves in the trays, (known as "weeping"), and short-circuit the flow patterns which are intended to maximize liquid/gas contacts.
Fixed deflector designs merely place a cover over a perforation in the tray deck to prevent liquid falling on to the perforation with sufficient velocity to penetrate even when the pressure is at design levels and while design features can minimize this risk, it can not be totally eliminated at low pressures.
In a typical movable valve, a perforation (for example a round hole) is stamped out of the material of the tray. This is then typically covered by the stamped-out shape, (a metal disc in the example), supported on legs or perhaps in a cage where the valve is meant to open and close with the gas pressure.
In a typical valve construction, the cover is made from the material of the tray. As will be seen, in typical valve constructions the dimensions or the aperture dictate the dimensions of the cover which, because the cover is usually formed from the material removed when the aperture is cut. It is for example impossible, using the material removed when the aperture is cut, a quadrilateral cover that will completely cover the aperture. It is also impossible to shape the orifice to create a venturi effect by punching through the material of the tray from the top to create a relatively narrow orifice opening at the bottom which opens into a wider channel at the top surface of the tray. It has been found that venturi effects are often desirable features of fixed valves.
The disadvantage of such prior art methods is that the shape of the perforation dictates the shape of the cover. There are therefore limitations that are inherent in the production process. The present invention provides a way of making mass transfer contacting devices in which the shapes of the perforation and cover can both be manipulated to produce the optimum process advantage and efficiency of operation of the device.
The present invention provides a simple mass transfer contacting device that can easily be installed and which provides a highly effective means of contacting liquid flowing over and around the valve with gas flowing up through the valve. Equally importantly, the design is such that the size and shape of the perforations and the covers can be independently optimized for the specific application. In addition the covers can be sized to fit any perforation.